The invention relates to a video output channel architecture. More particularly, the invention relates to a video output channel architecture which can be configured to meet different display standards.
A known video output channel architecture is shown in FIG. 1. In this architecture, the various parts of the composite video waveform are derived from three separate sources. The first is a digital video memory 1, which stores all of the information necessary to form the picture portion of the video waveform. As the memory 1 is scanned, addresses are presented to a video look-up table 3, which outputs three data values corresponding to the red, blue, and green color values for the current pixel. These digital values are then converted into analog signals by a video digital-to-analog converter 5. Video amplifier 7 then amplifies the analog signals to a level acceptable to drive the display.
The second source of information for the composite video waveform is a blank control signal generated by controller 4. The blank control signal instructs the digital-to-analog converter 5 to ignore the input data and output a zero level, and thus controls the blanking portion of the video waveform. The third source of information is the sync control signal, also generated by controller 4. The sync control signal controls application of a current source 4a to the green channel of the video output, and thus adds the appropriate sync level to drive the display.
A "snap-shot" of a typical video waveform is shown in FIG. 2. The levels of different portions of this waveform are important for conformance with industry display standards. Level A of the waveform in FIG. 2 is the maximum video level, i.e., the maximum of the active video region above the blanking or black level; the blank and black level can be at the same level for certain video DACs used. Level B is the composite sync level of the waveform, and level C refers to the blanking level. Each of these levels must be within certain specifications in order to meet a given display standard. Also important to conformance with a given display standard is the characteristic impedance of the output channel. The specifications for three representative display standards are shown in the table below:
______________________________________ Specifications A B C Impedance ______________________________________ RS170 (Mono) 1.0 0.4 0.075 75 RS170 (Color) 0.70 0.28 0.0525 75 RS330 0.714 0.286 0.0535 75 ______________________________________
According to the prior art, a video output channel such as that shown in FIG. 1 must be designed specifically to meet one of these industry standards. Thus, if it is desired to use a different standard, i.e., to use a different video display, a different output channel architecture must be used. Clearly, this is cumbersome and prevents swift reconfiguration of a video system.